by Jacob Wilson
The Mechanics of Bone Tissue
Part I
by
Jacob Wilson
It has ability to withstand thousands of pounds per square inch, to absorb extreme measures of shock, and to act as impenetrable Armour. Bone mechanics are indeed a fascinating subject. Moreover, they are an integral aspect in the career of each and every athlete. Presented before you is a comprehensive guide to understanding your skeletal system. Think of it as the foundation upon which further your body will be built, and most importantly be prepared to witness the awe inspiring world of a mechanical wonder.
Before delving in, Id like to explain the format of this dissertation. I have organized it into sections, and subsections. I have done so, with the intension that each athlete can come back and systematically review any question he or she might have in regards to their personal body type. I say this, because several concepts are addressed to specific audiences, including age groups, and gender. When such concepts appear, they will be clearly highlighted. Finally, this has been placed in a question and answer format to better your comprehension.
What Is Bone Tissue Composed Of?
The cell is the basic unit of life, where as a tissue is a large group of similar cells. In this particular case, the cells that make and secrete the material, which forms bone, are osteoblasts and osteocytes( 1, 6, 21, 25, ).
Let me break this down for you. The word osteo refers to bone. I.E. Osteology is the study of bone. We have two distinct suffixes. The first is blast which means young and the second is cyte, which refers to maturity. Thus, osteoblasts are young bone cells, and osteocytes are their older counter parts( 36 ). Consequently, this does have importance if one is to truly understand the nature of this tissue. Before delving into this however, I need to present a few guidelines to understanding composition.
1. Unlike skin, bone is not highly cellular( 21, 24, 25, 36 ). In fact, these units of life only compose two percent of the entire tissue, and yet are responsible for its entire structure!
2. Osteoblasts and Osteocytes secrete what is
called the matrix (4, 6). What is the matrix Neo? Simply put, it is all
material, which is outside of the actual cells. It is therefore by definition,
extra cellular material. Two main substances are secreted by the cells to
make this up. The first is a gel like material deemed the
ground substance, which for all intensive
purposes a mixture of proteins, sugars and water. The proteins, likened to
gelatin, give the mixture a viscous( thick )
like composition. Fibers are a second, and might I add extremely vital aspect
of this version of connective tissue. I say connective tissue, because that is
what bone is classified as( 6 ). There are several variations to CT. We like
to think of it as only tendons( connects muscle to bone), ligaments( connects
bone to bone ) and deep fascia( helps hold down muscles ). But it actually
comprises quite a bit more than this. CT, can be thought of as those aspects of
our bodies which literally connect other structures. The skeleton obviously
fits in this category( see notes at the end of this question ).
To digress, the fibers secreted by b. cells are mainly
collagenous( 43 ). Collagen, is a fiber
that I would highly recommend being familiar with. It is the fibrous protein
which provides all of your connective tissue with its extreme levels of
strength. Strength which you will witness first hand, as we explore the
mechanics of bone further into the article( when I say fibrous I mean stringy in
a sense, only these are comparable to steel in their ability to cope with stress
)( 36 ). 99 percent of the fibers secreted by bone cells are of this
composition. And might I add, this tissue is jam packed tight with it.
3. Bone calcifies It is vital to understand that bone tissue houses
both organic and inorganic material( 17 ). Sixty five percent is inorganic,
while the remainder is the latter. The organic component, as discussed is
composed of protein fibers called collagen, ground substance( the gel like
material discussed ) as well as the cellular component. The first process to
the formation of bone is the laying down of the matrix. In its un-calcified
form, the matrix is called osteoid( 35 ). However, several factors which
will be discussed shortly lead to deposition of mineral salts into the osteoid(
18, 30, 31 ). These mineral salts are primarily calcium and phosphate. They
are tiny crystals, which pack remarkably close together throughout the matrix,
and around the osteocytes and osteoblasts. There are several questions which
follow from the above statements
· What biomechanical components do the mineral salts add to bone?
· What biomechanical components do the organic materials add to bone?
· Can these two components be manipulated through exercise and diet?
Each of these questions will be answered. But for now, I
want to make it clear, that these are the parameters that must be understood if
one is to enhance the physiological parameters of their skeletal system.
Notes:
Above, we discussed that bone was a form of connective tissue. The reasons for
this are threefold. ( 1 ) All forms of CT, differentiate from mesenchymal
cells. These cells have the ability to essentially turn( differentiate ) into
all the various CT varieties. ( 2 ) CT literally connects different aspects of
the body. The skeletal system interconnects and binds various movements,
tendons connect muscle to bone, etc. etc. Interestingly enough, blood is
considered CT. It connects the body, via circulation of nutrients, fluids and
as you will see from Mr. Kings dissertation on hormones, this fluid is vital for
communication. ( 3 ) CT is not highly cellular, meaning cells are spread out,
and the tissue is composed primarily of extra cellular components.
What is The Basic Architecture of Bone?
Bone must possess extreme levels of strength, and at the
same time be light enough for locomotive purposes.
It is for this reason that its structure is arranged in two distinct
ways.
Cortical Bone Whenever you see the word cortex or cortical, it is usually
referring to the outer layer of a structure. Therefore cortical bone is the
outer layer of the skeleton( 17 ). As you will see, this aspect of bone must
withstand the greatest forces, and therefore must be equipped to cope. It is
for this reason that this is deemed dense bone as well( 36 ). The name is
due to the fact that it is extremely high in mineral content and the layers
which make it up are compact, and very tightly notched. We need to step aside
for a second and analyze this structure more closely.
A. Cortical Bone is made up of what are
called a Haversian systems( 24, 25 ). In simple terms a Haversian system can be
thought of as a pillar which runs parallel to the long axis of a bone. This is
no different then any pillar which supports a building. The system however is
made up of several layers. An outstanding analogy is to think of a trees
structure. The tree runs vertically, and yet, if you look inside of it, you can
see rings. These rings hug each other, just as tree rings do. There are
several reasons for this particular architecture.
1. Just as any other cell, osteoblasts and osteocytes need nutrients
from the blood. Diffusion of these needed raw materials for life, can never
take place through a calcified matrix! This means that your DNA, must have a
code that allows these cells to live and yet be surrounded by a dense
crystallized material which blocks them from the very substances which give them
life. How can such a problem be solved? Your bone is programmed to create a
pattern of canals, which allow for the passage of nutrients. I will explain
this structure.
§ As stated, one Haversian system is parallel to the long axis of a bone. It is made of circular layers, or rings of bone. These layers are called Lamellae( 25 ). That word means little plate. Once again, imagine a tree trunk. In the very heart of the layers( the middle of them ), is a hollow canal. This canal is called the central Haversian canal( 25 ). What do you think you would find within this canal, if you were its designer? The answer is a major blood vessel. It is this vessel which supplies the cells contained within the matrix with nutrients. The question now, is how does one get nutrients to cells which are still within a calcified matrix?
§ The answer is shear genius. In each circular layer or lamella lies several small spaces( cavities ), almost like rooms carved out in a home. Think of them as living quarters for bone cells. Each space is called a lacuna( pleural is lacunae ).
§
Coming out from these spaces are canals, which ultimately lead to
the central haversian canal. Osteocytes have a cell body, and like spiders they
have what look like legs. These legs lie within the canals, and nutrients can
enter them, and pass into the cell body for manufacturing materials for the
bones structure, and for the cells own life.
Additionally, these legs or extensions, not only communicate with the blood
vessel, but with other cells( 36 ). When I say communicate, I mean one cell leg
of an osteoblast is connected to a neighboring cell. These connections are
called gap junctions. A gap junction is a connection between cells, which
allows materials from one cell to be passed to a neighboring cell. Therefore
cells can exchange nutrients with each other, and with the life giving blood
vessels.
2. The second reason for these layers is
structure. One layer or lamella is again composed of cells and bone matrix.
However, the collagen fibers which run in one layer run in one direction. Each
layers collagen fibers run in different directions. As will be seen, this has
great mechanical significance!
B. The second type of bone is called spongy bone( 56 ). It is similar
to compact, accept that it is, as the name suggests spongy. In other words,
there are several pores within the bone. The spaces are formed, because the bone
is organized into supporting struts, that interconnect. Thus, it is layered,
but not dense. Once again we can deduce some conclusions:
§ Spongy or cancelous bone is lighter then compact, but cannot take as much strain( 54 ). Thus, bones made of primarily cortical bone, have a greater risk of fractures. However, it is strong, and can and does strategically support the surrounding cortex. Again, this is discussed when analyzing specific biomechanics of this tissue.
§ The lightness of the bone, allows strength, with a lesser amount of overall weight.
§ Within the pores lies red bone marrow( site of blood cell synthesis ), blood vessels and connective tissue( 56 ).
§ There is a cavity within the center of your bones, which again lessens the weight the body must cope with. Yellow Marrow lies within the cavity, which is a site for fat storage.
We can summarize by stating that Bone is a bloody
tissue. It also houses several other vessels, such as nerves, and lymphatic
vessels( immune response etc. ). You can deduce that with a high nutrient
delivery, bone is designed to remodel itself quickly and efficiently (59).
Thus, training can drastically change the structure, strength and ability of
this tissue (59). Additionally, an athlete who breaks or fractures a bone can
potentially recover and actually have greater integrity then before the injury
(53).
Finally, I would add, that bone is actually an organ. This is a structure that
has more then one tissue associated with it. Bone contains bone tissue, as well
as blood, fat and other significant structures. It can be affected by both
training, nutrition and hormonal environment, all of which is addressed in this
article (51).
Are There Any Specific Anatomical Aspects of The Skeleton That Would Enhance My Understanding of Its Function?
The answer to that is a definite yes.
1. The first aspect I would be familiar with is the basic organization that your bones are placed. Over 200 bones make up your skeleton, each are linked together by articulations( joints ). You have two basic sections to consider. The first is the axial skeleton or the frame work of your body. This consists of the vertebral column( backbone ), the skull, sternum, ribs, and hyoid bone( a bone in the neck region ). You then have the axial skeleton, which consists of the upper extremity, and the lower extremity, and the girdles, which connect them to your axial skeleton.
The girdles, like a belt are the clavicles( collar bone )
and scapula( where your shoulder blades are, and the Os Coxa( this bone is
actually thought of as the pelvic region, or the hip bone, and is in reality
composed of three bones ). The upper arm is known as the humerus, the forearms
which are composed of the radius and ulna are known as the forearm, and the hand
is known as, well, the hand( its composed of carpals, metacarpals and phalanges,
but thats another story ). In the lower body, you have the thigh region, in
which the bone is called the femur, and the leg is composed of two bones known
as the tibia( big thick bone where your shin sticks out ), and the fibula, and
finally the foot region.
What I have just done is summarize the areas, which we will look at in future
issues. It is important now to realize these regions for a greater orientation
and understanding of the body.
2. The second aspect I would be familiar with is the basic gross(
viewed without a microscope ) structure of a long bone. Long bones are just
that, longer then they are wide. Your humerus( upper arm ) is a long bone. The
shaft is called the Diaphysis and the ends of the bones are called the
epiphysis.
3. Thirdly I would understand the basic landmarks
that are common on a bone, and why they appear. This is very much related to
mechanics. Lets have a look.
- As stated there are 200 bones in the body. These bones form joints. On many
of these joints, you will find that the ends of the bones are rounded. This
rounded aspect forms a joint with the rounded aspect on its neighboring bone.
The definition for this is a condyle. Interestingly enough, joints are
classified by this very name. For example, there are two condyles on the femur(
thigh bone ), and two on the tibia( shin bone ). They form a joint which is
called a bi-condylar joint. Makes things rather simple does it not?
- The pre-fix epi, means above. Processes above
the condyles are so named epicondyles. You can palpate( feel ), two of these
right now. Your humerus has two condyles, above these are the lateral and
medial epicondyles. Take a second to feel to the mid aspect of your forearm,
near the junction of the forearm and upper arm. You should feel quite a large
bump, that is the medial epicondyle, the lateral epicondyle is on the opposite
side of the arm. These form as a result of muscular pull. This is because
tendons of the forearm muscles originate here. In fact the common flexors of
the forearm origionate on the medial epicondyle and insert on the wrist and
fingers. The lateral extensors originate on the lateral epicondyle and insert
on the wrist and fingers. This is a point I have made in the past, but I will
make it again. Notice that the medial protrusion is larger then the lateral.
This is due to the flexors being stronger then the extensors. Which should give
you a large hint that bone growth is quite manipulateable(59)! A concept that
we will expose for peak performance.
- On several bones you will find extremely shallow and smooth surfaces. Once
such surface can be found on your ulna( forearm bone ). Essentially these are
spots that regularly interact with other bones. Bend your elbow. Take a look
at that huge protrusion on the back of it. That is called the olecranon
process. Inside of that process, is a hollow area and when you extend your
elbow it interacts with the back of your upper arm. This is called the
olecranon fossa.
- Many bones have what are called facets. These are simply smoothed out areas
where joints are formed. As you will learn, most joints in the body are
synovial (25). These must be frictionless, which is partly why the area on
the bone is so smooth( it used to be capped with a glassy cartilage known as
hyaline ).
- Tuberosity When
bones are first formed, they are rather smooth. However, tendons and ligaments
have their attachment sites on your skeleton. When powerful muscles attach then
they pull the bone out into a rather pronounced bump (explained shortly). Reach
down and feel the huge bump on your frontal shin. That is called the tibial
tuberosity. What powerful muscle do you think attaches there?
The answer is the quadriceps femoris. Once again, we have empirical evidence
that bone can indeed be manipulated.
How Does Bone Grow, and What in the Heck is A Growth Plate?
Turnover A
physiological phenomenon that must be clearly understood. This refers to the
process by which a substance is broken down, and subsequently built back up.
For example, aspects of your muscle tissue are degraded (broken down into amino
acids), while at the same time new muscle tissue is synthesized (amino acids
linked together to form new muscle proteins). If the rate of synthesis
outweighs the rate of degradation, growth occurs, if degradation outweighs
synthesis a loss of tissue is the result (44, 53). Finally, maintenance is
reached when the breakdown rate = the build up rate.
Dynamic Such a concept refers to any physiological parameter, which is
constantly changing. I.E. Weight lifting is dynamic, as concepts such as torque
affect your mechanical advantage during a lift (52).
Both terms are vital to understanding your skeletal system. I say this, because
bone tissues turnover rate is extreme (20, 47,). For the average human being,
a total of six percent of your bone mass is replaced within a seven day time
period (36)! We have once again pointed out a concept of much consequence to
your training (3, 9, 14, 18, 20, 59).
There are two specific forms of growth. The first is
called appositional and the second is longitudinal( 25 ). The former is growth
from the outside of a tissue( an increase in width and diameter )( 35 ), the
second refers to growth from within a tissue( which would lengthen it ). It is
an interesting notion, that the latter is actually not possible. You see,
within a bone, once a cell has laid its matrix around itself, so that it is
fully contained in a cavity( lacuna ), it can no longer divide. This is because
division would cut off the osteocytes source of nutrients. However Mitosis(
cell division ), can occur on the outside of bone. This is where osteoblasts
come into play. They reside in incomplete haversian systems, meaning that a
full cavity has yet to be constructed, and thus mitotic division remains
possible. The blast suffix changes the minute the haversian system for a
particular group of cells is complete, and they are then considered mature
cells.
Longitudal Growth Analyzed How then, can
individuals grow in height? The answer is the much needed growth plate. When
you are developing in the womb, your skeleton is mostly constructed of
cartilage. This form of tissue does have the capability to divide internally.
As development furthers, the ends of the bones( epiphysis ), and the
shaft(diaphysis ), are replaced with bone tissue through a complex process known
as endocondral ossification( chondral refers to cartilage, ossification refers
to bone formation). However, between the junction of the epiphysis and the
diaphysis the cartilage remains. Thus, it divides from within, producing a
lengthening effect of the bone. The body is programmed to replace the cartilage
essentially as fast as it grows with bone, ensuring that the plate stays the
same size until closure.
I want you to consider that last point, and the design implications it entails.
If the cartilaginous plate continued to grow in size, a person going through
puberty would be literally standing on shaky ground, as this is not the tissue
you want to support your weight. The effects would be deleterious. Your
genetic code however is pre programmed to correct for this very case!
The main players involved in determining growth are hormones such as GH( growth hormone ), thyroid hormone, and androgens such as testosterone. As these increase during puberty, a massive growth spurt follows, as the cartilage is stimulated to divide. However, as the hormones continue to increase, they signal plate closure, and longitudinal growth ceases. Such concepts are of extreme importance to the young athlete, and I have provided an in depth discussion specifically for them within dissertation three of this series.
Appositional Growth
Osteocytes as you no doubt have figured out, are there to maintain bone matrix
(1). It is the blast variety that is responsible for growth on the outside of
the tissue. Before continuing I need to introduce one more player into the
game. The osteoclast. This is a bone- destroying cell (5, 14,). Why have
such a critter you ask? Bones are not simply for structure and protection, but
they are the bodys storage department for both phosphorus and calcium (48). As
described by Seksi, calcium is the key to all nerve conduction signals and
muscular contractions (48). Without it, you would not have the ability to take
your next heartbeat! Indeed, the body will, under all circumstances keep its
plasma Ca++ levels at an even keel, even if it means leaching those stores from
the bones. That is where nutrition comes into play. Bone is in a continual
state of flux. It is broken down all the time and replaced just as often. The
often aspect however can change. Meaning you can increase the rate of growth,
or decrease it( a net loss in bone ).
When bone is broken down by osteoclasts, the process is know as resorption,
while the increase is simply known as bone formation. The former process must
occur before the latter. As you will see, both are subject to
Wolfs Law( 59 ).
Conclusion
This ends part one of this series. The second article in the series covers the mechanical aspects of this tissue.
Yours In Sport
Jacob Wilson
Trainer@abcbodybuilding.com
President Abcbodybuilding / Hyperplasia Magazine
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